Engines incorporating a mechanism for recirculating an exhaust gas in an intake side to reduce fuel consumption of automobiles (EGR: Exhaust Gas Recirculation) have been introduced in the market. The aim of recirculating the exhaust gas is to reduce any power that the piston works (pumping loss) by reducing a manifold vacuum pressure (the difference between a cylinder internal pressure during an intake stroke and the ambient pressure) under the condition that the engine output power is small and to reduce the exhaust loss by suppressing abnormal combustion (knocking) under the condition that the engine output power is relatively large.
In order to remove a pumping loss under the condition that the engine output power is small, it is desired to recirculate the exhaust gas until the intake pipe pressure reaches the ambient pressure. However, since the exhaust gas basically serves as an inert gas, it is known that a flame propagation rate is reduced as the recirculating exhaust gas increases. In addition, if the recirculating exhaust gas amount reaches a certain level, the combustion becomes unstable. For this reason, there is an upper limitation determined under the condition of recirculating exhaust gas amount by which combustion stability can be secured (the combustion stabilization limitation mass). This upper limitation defines the recirculating exhaust gas amount considering and satisfying the combustion stability different depending on an engine type or a driving condition.
Meanwhile, the moisture of the air also serves as an inert gas similar to the exhaust gas to reduce the flame propagation rate. For this reason, if the recirculating exhaust gas amount is determined without considering moisture of the air, the engine combustion state becomes unstable depending on a humidity condition, and it may be difficult to obtain desired performance. As a technique for such problems, an internal combustion engine control device is discussed in PTL 1. This internal combustion engine control device computes a moisture amount of the fresh air on the basis of the detected humidity (relative humidity) and the fresh air target amount introduced into the engine cylinder and performs correction for reducing the exhaust gas amount as the moisture amount of the fresh air increases depending on the computed value.